1. Field of the Invention
The invention relates to telecommunications, and more particularly, to a digital subscriber line interface between a telephone line and central office equipment of a network operator providing voice and digital subscriber services.
2. Description of the Related Art
The demand for more bandwidth on existing conventional telephone lines increases as more customers send and receive digital data over telephone lines. In general, a conventional telephone line is comprised of a pair of copper conductors that connect a telephone set to the nearest central office, digital loop carrier equipment, remote switching unit or any other equipment serving as the extension of the services provided by the central office. This pair of copper conductors, which is also referred to as a twisted pair, has its leads named as tip and ring. The tip and ring nomenclature is derived from the electrical contacts of an old-style telephone plug. Replacing these existing copper telephone lines with high-speed fiber optic cable is often found to be cost prohibitive. Additionally, an optical cable cannot deliver the electrical power necessary to run the voice and signaling circuitry of the connected equipment such as a conventional telephone set. The central office equipment, therefore, must provide such power. Moreover, substantial investments in the existing copper infrastructure by network operators generally necessitate continued use of that copper infrastructure. Thus, broadband technologies that can utilize existing copper telephone lines and co-exist with conventional narrowband analog phone service are highly desirable.
One such technology is the conventional integrated services digital network (ISDN). ISDN boosted transmission rates over existing copper telephone lines to 128 Kilobits per second (Kbps). Although voice data can be transmitted over an ISDN line, special terminal adapters that require an external power source must be provided in order to do so. When these requisite special terminal adapters fail because of a power outage, the ISDN line is rendered nonfunctional. This undesirable trait of ISDN lines is a particular problem should an emergency situation arise and a power outage has rendered the ISDN line useless for voice communication. In such a situation, lifeline telephone service would be disabled. As such, most ISDN lines are used in conjunction with a Plain Old Telephone Service (POTS) line. POTS lines are immune to power failures for extended time periods with back-up battery power provided by the phone service provider. Competing technologies that do not require a dedicated second line for failsafe voice telephony, therefore, are generally preferred over ISDN technology. Moreover, such competing technologies provide transmission rates superior to those offered by ISDN technology.
Another broadband technology that can utilize existing copper telephone lines is the digital subscriber line (DSL). Several variations of DSL technology are being developed, such as High-bit-rate DSL (HDSL), ISDN DSL (IDSL), Symmetric DSL (SDSL), Rate Adaptive DSL (RADSL), and asymmetric DSL (ADSL). ADSL is particularly attractive for consumer Internet applications where most of the data traffic is downstream towards the subscriber. For detailed descriptions of ADSL technology, see U.S. Pat. Nos. 5,461,616, 5,534,912, and 5,410,343.
Some DSL technologies, such as ADSL, have the advantage that ordinary voice data transmissions can share the same telephone line with digital data transmissions. The lower frequency band of the telephone line is used for voice data, while the upper frequency bands are used for digital data. However, because each of these frequency bands operates on the same phone line, DSL technology employs a splitter to isolate the signals of each band. In some applications, this splitter is comprised of two passive filters: (1) a low-pass filter that isolates the low-frequency components (POTS data) of the transmission, and (2) a high-pass filter that isolates the high-frequency components (digital data) of the transmission. The splitter also operates as a mixer to combine the high-frequency digital data with the low-frequency voice data, and provides the combined signal to the telephone line.
A problem with the DSL approach is that the passive elements (e.g., inductors and capacitors) that can be used to implement the low and high pass filters of the splitter are bulky and cumbersome. This added bulk requires more physical space to house the splitter, and consequently occupies more physical space in the central office of the telephone company. In the aggregate, the physical space consumption resulting from this approach is costly. Also, this approach is associated with additional material and manufacturing costs involved in building the splitters, as well as costs associated with deploying those splitters.
Another broadband technology that can utilize existing copper telephone lines to perform both the voice and digital data communication functions is the use of a silicon broadband subscriber line interface circuit (SLIC). This approach provides an active discrete splitter that is more compact than a passive splitter. However, such technology requires a high-voltage, high-speed silicon process and consumes significantly more power thereby limiting the service circuit line density. Moreover, this approach requires an additional power backup in order to sustain the supported communication channels for the same time period compared to that of a POTS only service in the event of a power failure.
Thus, what is needed is an interface between a telephone line and the central office that can receive and isolate low frequency POTS data and high frequency digital data from the telephone line. The interface should also be able to combine and transmit low frequency voice data and high frequency digital data onto a telephone line. The interface should be compact while at the same time provide the requisite filtering, and should have low-power consumption. Additionally, the interface should provide a POTS line that is immune to power failures.